This invention relates to a load cell for converting the magnitude of an applied load into an electrical signal. More particularly, the invention relates to a load cell designed so that strain gauges and compensating elements can be connected thereto with facility.
As shown in FIG. 1, a conventional bending beam-type load cell includes a stationary portion 1 for affixing the cell, a portion 2 for receiving an applied load, and parallel, flexible beams 3, 4 joining the fixed portion 1 and load-receiving portion 2. The beams 3, 4 include portions of reduced thickness and have strain gauges C-1, T-1, C-2, T-2 affixed thereto at respective ones of the reduced-thickness portions. As shown in FIG. 2, the four strain gauges C-1, T-1, C-2, T-2 are connected to form a bridge, a constant voltage is applied across the opposing junctions a, b through a feeder resistor Ni, and an electrical signal corresponding to the magnitude of a load applied to the load cell is obtained from across the other pair of opposing junctions c, d. More specifically, when a load is applied to the load-receiving portion 2, a potential difference develops across the junctions c, d owing to a change in the resistance of the strain gauges C-1, T-1, C-2, T-2, which measure the strain developed in the parallel beams of which load cell, the strain corresponds to the magnitude of the applied load. Reading the potential difference from output lines G, B makes it possible to detect the magnitude of the load.
Following assembly, the load cell 5 is adjusted for bridge balance and is subjected to a zero-point temperature compensation. These operations are performed by measuring the output of the load cell 5 in the unloaded state, checking the measured value against a reference value to determine when the former is within allowable limits and, when the measured value falls outside of the allowable limits, inserting a compensating lead wire in series with prescribed ones of the strain gauges in the bridge circuit, this being decided in accordance with whether the measured value deviates toward the positive or negative side. For example, when there is a positive deviation caused by poor bridge balance, a compensating Constantan wire 6 is inserted in series with the strain gauge C-2, as illustrated in FIG. 3. On the other hand, if a zero-point deviation attributed to temperature indicates a shift toward the negative side, then this is compensated for by a fine copper wire 7 inserted in series with the strain gauge T-2, as shown in FIG. 4.
The foregoing is shown in greater detail in FIG. 5, illustrating one example of how compensation is made. A terminal board 8 for effecting connections is attached to the load cell and is equipped with terminals 8a through 8i for connections to the lead wires from the strain gauges and to input and output leads R, G, B, W. The compensating Constantan wire 6 and compensating copper wire 7 are connected in series with the proper strain gauges according to the direction of deviation caused by poor bridge balance or temperature-induced zero shift, respectively. In the illustrated example of FIG. 5, the compensation is for a bridge balance shift on the positive side and a temperature-induced zero point shift which is also on the positive side. Thus the Constantan wire 6 is inserted in series with the strain gauge C-2 and the fine copper wire 7 is inserted in series with the strain gauge C-1. In general, the Constantan wire 6 is inserted in series with strain gauge C-2 for a positive bridge balance deviation and in series with strain gauge T-1 for a negative bridge balance. The fine copper wire 6 is inserted in series with strain gauge C-1 for a positive zero-point deviation caused by temperature, and in series with strain gauge T-2 for a negative zero-point deviation.
When correcting the load cell 5 through the foregoing means, the strain gauges C-1, T-1, C-2, T-2 are first connected directly to the external input and output leads R, G, B, W, a measurement is taken with the cell in the unloaded state, the magnitude and direction of any deviation, caused by bridge balance and a temperature-induced change in the zero point, is checked by using the measured results, and proper connections for effecting compensation are determined based on the preceding step. Then, strain gauge lead wires are disconnected from terminals 8c, 8e of the terminal board 8 by melting the solder joints and the required Constantan wire 6 or copper wire 7 is inserted be being soldered across the terminals 8e, 8d or 8b, 8c, respectively. Since the foregoing steps are performed within the very limited confines of the temporarily assembled load cell, operability is extremely poor and wiring errors are likely to occur.
A known improvement over the foregoing is illustrated in FIG. 6. A set of three terminals a, b, c is provided at the connection between the strain gauges C-1, T-2 and the output lead B, and at the connection between the strain gauges T-1, C-2 and the output lead G. The output leads G, B are connected to respective ones of the centrally located terminals a, a, the ends of strain gauges T-1, C-1 are connected to the terminals b, b and the ends of strain gauges C-2, T-2 are connected to the terminals c, c. Short-circuiting leads 9 are connected between each central terminal a and the terminals b, c on either side thereof before the load cell is assembled. The arrangement is then provided on the beams 3, 4 (FIG. 1) with the strain gauges located at the proper positions to construct the load cell, after which the cell is subjected to measurement in the unloaded state in order to adjust the bridge balance. Thus, the initial connections are such that each of the strain gauges C-1, T-1, C-2, T-2 is connected to the output leads G, B by the short-circuiting leads 9. Conducting a measurement under these conditions makes it possible to determine the magnitude of a deviation caused by poor bridge balance or a change in zero point attributed to temperature. When the measured value falls outside the allowable limits relative to a reference value, the location at which a compensating lead wire should be inserted can be decided based upon whether the deviation is on the positive or negative side. The short-circuiting lead wire 9 at the proper position is then severed and removed and is replaced by inserting the Constantan wire 6 or fine copper wire 7. This completes the bridge balance adjustment of the load cell 5. A specific example of the conventional approach of FIG. 6 is as shown in FIG. 7, for a case where bridge balance indicated a positive deviation. A Constantan wire 6 is shown inserted across terminal a and terminal c, to which the strain gauge C-2 is connected. Also, to compensate for a negative deviation owing to a temperature-induced shift in zero point, a copper wire 7 is inserted across terminal a and terminal c, to which the strain gauge T-2 is connected.
FIG. 8 shows specific example of the wiring associated with the conventional method employed in FIGS. 6 and 7. Intermediate terminal boards 10a and 10b are for connecting the leads of the strain gauges C-1, T-1 and C-2, T-2, respectively, and for this purpose each comprises a printed circuit board having plural connection terminals 11 and wiring 12 formed thereon. A terminal circuit board 13 for the external input and output leads includes a circuit board on which there are formed plural connection terminals 14 for connection to the intermediate terminal boards 10a, 10b, plural terminals 15 for connecting external input and output leads, the two sets of terminals a, b, c for connecting the Constantan wire 6 or fine copper wire 7 (FIG. 7) and the short-circuiting lead wires 9, and the wiring 16.
With the conventional arrangement of FIG. 8, the intermediate circuit boards 10a, 10b are disposed on the load cell beams 3, 4 intermediate the respective strain gauges C1, T-1 and C-2, T-2, and the terminal board 13 is affixed to the stationary portion 1 of the load cell. An adjustment is carried out with the load cell 5 in the assembled state by measuring the output of the load cell in the unloaded condition, determining bridge balance and zero-point deviation from a reference value on the basis of the measured value, cutting out a maximum of two of the short-circuiting lead wires 9 from the proper positions when compensation is required, and replacing a removed lead wire 9 with the Constantan wire 6 or copper wire 7.
According to the method illustrated in FIGS. 6 through 8, the short-circuiting lead wires 9 are connected beforehand to the required points on the easily-assembled, separate printed circuit board 13, and the printed circuit board is affixed within the load cell. When it is necessary to compensate for poor bridge balance or a shift in the zero point due to temperature, a maximum of two of the short-circuiting lead wires 9 are severed and removed from the proper locations, and the Constantan wire 6 or fine copper wire 7 (or both) is inserted in place of the removed short-circuiting wire(s). The adjustment procedure is thus simplified and is less likely to invite wiring error.
In affixing the terminal board 13 to the load cell proper, the terminal board is attached in a bent condition because of limitations imposed by the dimensions of the load cell and the terminal board. While the terminal board itself bends freely owing to its flexibility, the electrical wiring patterns, formed of a metal foil such as copper foil, tend to break or sustain other damage owing to such bending. When breakage occurs, open points which develop in the wiring patterns from such breakage must be mended by connecting a lead wire across them. The repair work is extremely difficult, however, as it must be performed in a small amount of space. It is also very difficult to locate where the lead wires should be connected.
Another disadvantage with the above-described arrangement is the high probability that wiring errors will be made during an adjustment. The reason is the large number of terminals on the terminal board, and the fact that the location at which the Constantan wire is inserted differs depending upon the direction (positive or negative) of the bridge balance deviation. The same is true with regard to the insertion of the fine copper wire to compensate for a shift in the zero point caused by a temperature. Wiring errors are likely not only during adjustment but also when the strain gauges are connected to their respective terminal boards and when connections are made between terminal boards. Again, the reason is the multiplicity of terminals. To avoid errors during initial wiring and later adjustments, therefore, practice has been to refer to a wiring instruction manual during these operations. This makes wiring work very troublesome, however, and diminishes operating efficiency.